Typically, the clutch in an automobile or a light truck is controlled by a clutch pedal connected through a linkage to the clutch throwout bearing. However, when the clutch must transmit relatively high loads such as in heavy duty trucks and construction equipment, the force required to move the clutch pedal is sufficiently high to warrant power assist devices. In addition, for some applications it is desirable to eliminate the rigid linkages typically utilized between the clutch pedal in the vehicle cab and the clutch. For example, in the "cab over" truck the linkage must be flexible or detachable for servicing purposes. Therefore, a fluid actuated operating mechanism can be utilized to actuate the clutch with reduced pedal effort in response to control of the driver through the clutch pedal and allow the use of a flexible cable or the like between the cab and the clutch. The fluid utilized in such applications is preferably pressurized air and is so referred to hereinafter although it will be understood that any gas or liquid can also be used.
Normally, clutch operating mechanisms are connected to a pivoted link to transmit force to the clutch throwout bearing. Most clutches have a bearing load curve that increases from full engagement to full disengagement and are readily controlled. Some clutches, have a lower bearing load at full disengagement than at some point between full engagement and full disengagement. The prior art air actuated clutch operating mechanisms are not satisfactory in operating such clutches.
The prior art air actuated operating mechanisms can be of the type which are operated from a source of vacuum, such as the intake manifold of the vehicle engine, or of the type which are operated from a source of air under pressure where a relatively high force is required. The operating mechanism requires a valve for connecting to and disconnecting from the source hwen it is desired to actuate the clutch.